What restriction applies when planning to use an alternate airport that has a closed control tower?

When planning to use an alternate airport with a closed control tower, one significant restriction is that you cannot use any instrument approaches to qualify that airfield as an alternate. The reason for this restriction is that closed control towers indicate limited or no air traffic services, which compromises the ability to execute instrument approaches safely.

In instrument flying, the approaches are designed to ensure safe landings under various weather conditions, and functioning control towers play a crucial role by providing essential services such as traffic advisories and approach clearances. Without these services at an airport with a closed control tower, pilots are unable to rely on instrument procedures, which are vital for maintaining safety and order during approaches and landings.

Given these factors, it becomes clear why an airport with a closed control tower cannot serve as a qualified alternate based on instrument approach procedures, thus making it a crucial point for pilots to consider during their flight planning.